1. Field of the Invention
This invention relates in general to a method to detect and report object displacement utilizing optical triangulation principles, and more particularly to a method and apparatus to detect and report object displacement within a fixed displacement window defined by a minimum and maximum displacement or within a window having a fixed size about a midpoint.
2. Description of Related Art
It can be seen that there is a need for a method and apparatus for determining the displacement of an object which possesses a linear relationship between a pair of user defined minimum and maximum points of displacement.
It can also be seen that there is a need for a method and apparatus for determining the displacement of an object which can be easily computed while providing a method for controlling the output signal generated in response to the measurement of the object displacement.
A pictorial overview of a system for optically determining object displacement is illustrated in FIG. 1. As shown in FIG. 1, the sensor is a device based on optical triangulation. A light emitting element 101, such as a laser diode, light emitting diode, or similar device, transmits light in the direction of motion in which the object's relative position is to be gauged. A conditioning element 102, such as a lens, can be used to focus the light from the source to enhance the performance of measuring device. The light from the emitter interacts with the target 103, which in turn scatters a small portion of the light in the direction of the receiving element 104, such as a CCD array, PSD, or other linear detection array element. A single lens or lens system 105 may be used to enhance the collection of the light scattered from the target to increase the performance of the device. The information generated by the receiving element is conditioned with analog 106 and digital electronics 107 and is used to calculate the position of the light on the receiving element. This generated information is then used to calculate an output which is linearly proportional to the target's position within a set of sensing limits, called the Minimum Sensing Distance 110 and the Maximum Sensing Distance 111.
Existing implementations of displacement systems in this sensing area can be characterized primarily into two areas: 1) the type of transfer function used to map the position of a target from information generated by the receiving element, and 2) the implementation of the transfer function. Primarily, designs of these existing systems provide an absolute distance measurement, an output proportional to a fixed sensing range, or an output proportional to a fixed sensing range with limited electronically implemented adjustment to the scale of the analog output which provides slight adjustments in the slope or intercept of the output.
Optical displacement sensors based on the established principle of triangulation have been sold commercially for many years. What distinguishes displacement sensors from discrete output sensors based on triangulation, which are referred to as adjustable field (or AF) or fixed field sensors, is a displacement sensor's ability to provide an analog output that is ideally linearly proportionate to the sensing distance. Examples of some of these types of sensors would be those manufactured by Keyence Corp. (LB70, LB1000, and LC series), Aromat Corp. (ANR, LM100/200, and LM 300 series), DynaVision.TM. (LDS and LTS series, Omron (Z4M series), Idec (MX1 series), and CyberOptics (DRS series). The outputs of these systems are primarily a value proportional to the target distance. Some of the systems (as with CyberOptics DRS series) output absolute distance from a reference point. For most of the other systems, an analog voltage or current is produced that allows users to indirectly measure the distance based on the sensor's known operating range. Several of the systems also provide for trimming the analog outputs using potentiometers or pushbuttons. This output trimming process is used to adjust the scale or offset of the output.
In a basic sense, all of the aforementioned sensors operate upon one principle; as a target in the sensor's field of view moves, it's image on a receiving element also moves. If an appropriate receiving element is used, for example a lateral effect photo-diode (PSD) or a linear CCD array, the change in position can be quantified and converted into an ideally linear analog output over the operating range of the sensor. This relationship is shown in FIGS. 2-4. In FIG. 2, the target moves from position A 201 to position B 202 and finally to position C 203. Assume that the distance the target moves from position A 201 to position B 202 is the same as the distance moved from position B 202 to position C 203. The light from an emitter element 210 is directed through a conditioning lens 211 or lens system toward the target. A small amount of light scattered from the target is imaged on the receiving element 212 through a lens 213. As the target moves from position A 201 to position B 202 to position C 203, the position of the image on the receiving element also moves. This movement across the receiving element is non-linear with respect to the distance the target travels as depicted in FIG. 3. This non-linear displacement relationship must be processed through a linearization process to create an output that is linearly proportional to the distance traveled as depicted in FIG. 4.
Linearization Techniques
The technique, and corresponding side affects, to implement the linearization process is the primary distinguishing characteristic of the present invention as compared to the current state of the art. Typically, one of three approaches is used for linearizing the output of triangulation based displacement sensors: 1) linearize the output signal through some configuration of electrical circuits, 2) linearize the output signal through the use of look-up tables that directly "store" image position data as a function of target distance, or 3) linearize the output signal through an appropriate transfer functions derived from the fundamental geometric relationships of the sensor.
1) Linearize With Electrical Circuits
One particular technique for dealing with the inherent non-linearities of a triangulation based sensors is to use electrical circuits to linearize the output over a pre-defined working range as disclosed in U.S. Pat. No. 4,761,546 issued to Ikari, et al. These types of techniques are inherently more susceptible to temperature effects, and thus requiring more circuitry to compensate for these undesirable effects, as well as are more difficult to achieve a high degree of output linearity over a large range of target motion.
2) Look-Up Tables
Another technique of dealing with the non-linear nature of triangulation based measurements is to do a point by point calibration of the system and then storing the position of the image on the receiving element for each associated sensing distance in look up table. As the target is moved within the sensing region, the output is adjusted according to the data stored in the look up table. Systems using this technique are described in U.S. Pat. No. 4,705,395 issued to Hageniers and U.S. Pat. No. 4,774,403 issued to Arts.
Systems that linearize based on a look-up table calibration can provide a high degree of linearity on the output. However, in order to achieve this high degree of resolution, a large quantity of measurements must be taken and subsequently stored. This is not efficient from a manufacturing aspect or when required to implement this approach in a real-time operating system. In addition, displacement information for positions not calibrated must be numerically interpolated which can be computationally intensive to make accurate.
3) Linearize Through an Transfer Function Defining the Geometry of the System
Several approaches have been proposed using a transfer function, which includes a direct system of governing equations, to linearize the output from the receiving element. These systems include those disclosed in U.S. Pat. No. 4,761,546 and U.S. Pat. No. 5,113,080 issued to Leu, et al. In the case of the former, the system is linearized for a specific geometrical configuration. The latter uses an exact system of equations based on a more arbitrary geometric configuration. The system of equations disclosed in Leu, et al. are derived with the intent to measure absolute displacement which is reflected in the form of the primary transfer function. In addition, calibrating the transfer function, as disclosed by this reference, relies on direct measurements of several important optical parameters. While the author claims that these parameters "can easily be measured"; in practice, this is almost certainly not the case.
The present invention attempts to overcome the deficiencies of the prior art as described above in the following manner. The invention provides an output which is ideally linearly proportional to a target's position within a set of user defined sensing limits. This feature is a significant improvement over other optical triangulation devices of similar design. The transfer function used to calculate the relative position of the target with respect the user defined sensing limits is of a form that allows it to be efficiently implemented within a microprocessor. The form of this function is unique when compared to inventions of similar function. The linearity of the transfer function over the user defined set of sensing limits can be maximized by adjusting a single constant. This provides for significant improvement over other inventions of similar design.